1. Field of the Invention
The present invention relates to an optical low-pass filter that is placed in front of a solid state image sensor having an offset sampling structure, for eliminating high-frequency components from incident light.
2. Background Arts
The solid-state image sensor has an array of photo sensor cells, called pixels, for obtaining an image signal through discrete sampling and photoelectric conversion of an optical image of a subject. According to the sampling theorem, higher frequency components than a Nyquist rate, that is one half of a spatial frequency determined by the pitch of arrangement of the pixels, cause the aliasing noise. Since the aliasing noise appears in the form of spurious signal or moirxc3xa9, it is necessary to eliminate the higher frequency components prior to the sampling. For this purpose, an optical low-pass filter is placed in front of the solid-state image sensor in an imaging apparatus.
As the optical low-pass filters, those utilizing double refraction or birefringence of the crystal are widely used. An exemplary of the optical low-pass filter is disclosed in Japanese Laid-open Patent Application No. 60-164719, which efficiently suppresses the higher frequency components of light incident on a solid-state image sensor having a square grid structure. This optical low-pass filter uses three crystal plates in combination, and splits the incident light ray into eight rays with equal intensity.
On the other hand, a solid-state image sensor having a so-called offset sampling structure has been developed and introduced in practice, for the sake of improving optical resolution. In the offset sampling structure, pixels of one row are shifted from adjacent rows in a horizontal scanning direction of the solid-state image sensor by an amount corresponding to half a pitch or sampling interval in the horizontal scanning direction.
FIG. 13 shows ideal frequency characteristics or ideal modulation transfer function (MTF) curves of the optical low-pass filter for the solid-state image sensor of the offset sampling structure where the pitch in the horizontal scanning direction is twice a pitch of the pixel in the vertical direction. In FIG. 13, xe2x80x9cfxe2x80x9d represents a frequency standardized by the pitch in the horizontal direction. The MTF is standardized such that the MTF value of the direct current component (f=0) is 1. The curve shown by a solid line represents the characteristics in the parallel or the vertical direction to the horizontal scanning direction, whereas the curve shown by dashed lines represents the characteristics in the directions of xc2x145xc2x0 to the horizontal scanning direction.
Because the ideal frequency characteristics of the optical low-pass filter for the solid-state image sensor of the offset sampling structure are different from those for the solid-state image sensor of the square grid structure, the optical low-pass filter for the solid-state image sensor of the square grid structure cannot efficiently suppress the aliasing noise. Japanese Laid-open Patent Application No. 3-46615 discloses an optical low-pass filter for the solid-state image sensor of the offset sampling structure, which uses optical members splitting the incident light in directions of xc2x145xc2x0 to the horizontal scanning direction, in combination. As shown in FIG. 14, the optical low-pass filter 40 of this prior art is composed of three optical members 41, 42 and 43 that are placed in front of a solid-state image sensor 44.
FIGS. 15A, 15B and 15C illustrate the splitting process of the incident light through the first to third optical members 41 to 43 of the optical low-pass filter 40, respectively. The first optical member 41 is a birefringent plate that splits an incident ray into an ordinary or rectilinear ray B1 and an extraordinary ray B2 that is refracted in the direction of xc2x190xc2x0 to the horizontal scanning direction. The second optical member 42 is a birefringent plate whose optic axis is inclined by 90xc2x0 to that of the first optical member 41, so the ray B2 travels straightly through the second optical member 42, whereas the ray B1 is refracted in the direction parallel to the horizontal scanning direction, and projected at a point B1xe2x80x2. The third optical member 43 is a birefringent plate that splits a ray in the direction of +45xc2x0 to the horizontal scanning direction by a split width of P2. As a result, the incident light is split into two rays in the direction of xe2x88x9245xc2x0 to the horizontal scanning direction by a split width of P1 through the first and second optical members 41 and 42, and thereafter split into four rays through the third optical member 43.
Where the split width P1 in the direction of xe2x88x9245xc2x0 and the split width P2 in the direction of +45xc2x0 are equal to each other, the optical low-pass filter 40 has frequency characteristic curves as shown in FIG. 16. As seen from these curves, the higher frequency components of the incident light are effectively suppressed in either direction, though it is incomplete in comparison with the ideal characteristics shown in FIG. 13.
Recently, a demand for minimizing the solid-state imaging device and thus making the apparatus handy and portable is increased. Since the conventional optical low-pass filter such as disclosed in the above mentioned prior arts needs three birefringent plates, it has been difficult to minimize the optical low-pass filter, and the conventional optical low-pass filter is relatively expensive.
In view of the foregoing, an object of the present invention is to provide an optical low pass filter for a solid-state image sensor of the offset sampling structure, that is compact and may be produced at a low cost, but suppress the higher frequency components of the incident light with high efficiency.
According to the present invention, an optical low-pass filter for a solid-state image sensor of an offset sampling structure comprises a first optical member that splits an incident light ray into a rectilinear ray and a refracted ray whose propagation direction is about 45xc2x0 or about xe2x88x9245xc2x0 to the horizontal direction; and a second optical member that splits an incident ray into a rectilinear ray and a refracted ray whose propagation direction is about 90xc2x0 or about xe2x88x9290xc2x0 to the horizontal direction.
Since the optical low-pass filter of the present invention needs only two optical members, it contributes to reducing the thickness and the cost of the solid-state imaging apparatus.
On the assumption that pixels of the solid-state image sensor are arrayed at a pitch Px in a horizontal direction and at a pitch Py in a vertical direction, and are shifted by an offset amount of Px/2 in the horizontal direction from the pixels of the adjacent rows, it is preferable to define respective split widths D1 and D2 of the incident light rays through the first and second optical members to satisfy the following conditions:
D1≈Px/{square root over (2)}
Pyxe2x89xa6D2xe2x89xa62xc2x7Py 
According to a preferred embodiment, the second optical member is bonded to a front side of the solid-state image sensor, instead of a conventional glass protection plate. Thereafter, the first optical member is mounted in front of the second optical member. Thereby, the thickness of the solid-state imaging apparatus is still more reduced.
It is preferable to provide each of the first and second optical members and the solid-state image sensor with a positioning notch or a positioning mark. By aligning the positioning notches or marks with each other, the first and second optical members are positioned properly relative to the solid-state image sensor without fail.